1. Field of the Invention
The present invention relates to a double-layer pellet which is to be placed in the high-temperature plasma generated in a fusion reactor or an experimental fusion apparatus, in order to refuel the reactor or the apparatus, to analyze particle transport, to improve plasma heating and to serve a similar purpose. The invention also relates to an apparatus and method for manufacturing the double-layer pellet.
2. Description of the Related Art
Fossil fuel, which is the main energy source we use today, will be exhausted in the future. Energy-acquiring technology utilizing nuclear fusion is being developed in various regions over the world, such as EC, Japan, the U.S. and Russia. The progress in this development is accompanied by two things. The first is an increase in the size of the experimental apparatuses employed. The second is an increase in the size of the plasma generated therein. Of the fusion reactors hitherto known, a D-T type fusion reactor fueled with deuterium and tritium is most promising in view of its reaction efficiency.
Generally known as a method of refueling the D-T type fusion reactor is gas puff method, in which deuterium gas and tritium gas are injected into plasma.
The gas puff method is, however, disadvantageous in some respects. First, the fuel particles can hardly reach the core of the plasma, particularly when the plasma is large. Second, it is quite probable that the fuel particles are repelled back by the peripheral part of the plasma, known as "divertor layer," before they are injected into the plasma.
To render the gas puff method more efficient, experiments have been conducted in which single-layer pellets made by solidifying tritium gas or a mixture of deuterium gas and tritium gas are injected into plasma.
Such single-layer pellets are manufactured by an apparatus generally known in the art in such a manner that liquid helium is supplied to the cryo portion cooling metal pipes or a metal plate having holes to an extremely low temperature; hydrogen isotope gas is supplied into the metal pipes or the holes of the cooled metal plate, and then the gas is solided in the pipes or the holes, thus forming single-layer pellets. New technique of manufacturing single-layer pellets has recently been developed, as is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-240102. This technique consists in forming a core of another hydrogen isotope within a single-layer pellet, thereby rendering the isotope-mixing ratio in the outer layer different from the isotope-mixing ratio in the core.
The apparatus produces pellets in which solid tritium is present in the surface. When the pellets are placed in the plasma generated in a fusion reactor or the like, tritium is not only supplied into the central part of the plasma which is at high temperatures and high density and in which nuclear fusion may take place with high efficiency, but also supplied, undesirably, into the peripheral part of the plasma which is at relatively low temperatures.
Tritium is a radioactive substance; it must be wasted as little as possible to minimize the load on the exhaust pump system, tritium-recollecting system and tritium-separating system of the fusion reactor. The tritium supplied to the divertor layer of the plasma is partly repelled from the plasma, and is not used at all in the nuclear fusion. Part of the tritium is adsorbed by the wall of the vacuum vessel of the reactor. The remaining part of the tritium is evacuated from the vacuum vessel by the vacuum pump system. This inevitably results in an increase in the load on the tritium-processing system.
To solve these problems which are inherent in the use of single-layer pellets, there have been invented double-layer pellets. A double-layer pellet is made up of a core of solid tritium and an outer layer of solid deuterium, which completely covers the tritium core. An apparatus for manufacturing double-layer pellets is known, as is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-335187.
This apparatus has metal pipes in which gases solidify to form double-layer pellets. In this apparatus, each double-layer pellet is made in three steps. First, deuterium-gas is supplied into each metal pipe, forming a hollow cylinder or layer of solid deuterium on the cooled inner surface of the metal pipe. Then, tritium gas is supplied into the deuterium cylinder, forming an elongated tritium core. Finally, deuterium gas is applied into the pipe from both ends thereof, forming two deuterium layers which cover the ends of the elongated tritium core. As a result, a double-layer pellet is obtained which consists of a tritium core and a deuterium layer completely covering the core.
To design an efficient fusion reactor it is necessary to know physical property of transport of particles and heat. This is because the size of the reactor greatly depends on the physical property of transport of particles and heat. Thus, the transport of particles and heat influences the cost of building a fusion reactor. This is very important from an industrial point of view.
Despite the efforts made over years in the past, the transport of particles and heat in high-temperature plasma cannot be said to be well understood. An active particle transport diagnosis is known, which serves to analyze the transport of particles and heat.
Known as two typical examples of active particle transport diagnosis are: laser blow-off method and impurity-pellet injection method.
In the laser blow-off method, a substrate of glass or the like, having a silicon or aluminum or the other atom film bonded to it, is placed in a plasma-confining vacuum vessel, and a laser beam is irradiated on the silicon or aluminum film, thereby evaporating the silicon or aluminum and leading the resultant vapor into the plasma, and the behavior of particles is observed. In the impurity-pellet injection method, an apparatus is used to inject impurity pellets into the plasma present in a plasma-confining vacuum vessel. The apparatus has pipes or a metal plate having holes or pipes. Pellets made of a solid material such as lithium are set in the pipes or the holes of the metal plate and injected into the plasma, propelled by high-pressure gas or the like.
The experimental fusion apparatus is equipped with no mechanism which can reliably determine the sizes of the outer layer and the core or the positional relation thereof.
The active particle transport diagnosis, described above, cannot serve to analyze the physical aspects of transport of particles and heat in a fusion reactor. More specifically, the diagnosis cannot provide a reliable quantitative analysis of spatial changes in particle transport. This is because the absolute number of particles supplied is unknown, particularly in terms of space, and the particles are deposited in all locations from the peripheral part of the plasma into the central part of the plasma.
There is another problem with a fusion reactor or an experimental fusion apparatus. The high-temperature plasma generated in the fusion reactor or apparatus contains tail ions produced by the ion cyclotron range of frequency (ICRF) heating method. If heated high-energy ions are present also in that region of the plasma in which energy loss is prominent, the heating efficiency inevitably decreases.
To solve the problems mentioned above and to analyze the physical aspects of transport of particles and heat into a fusion reactor, there is a demand for double-layer pellets which have the similar structure as refueling pellets, each of which comprises a core and an outer layer whose sizes and positional relation can be reliably determined, and which can serve to inject particles, in no excess numbers, to a desired part of plasma. For example, double-layer pellets whose cores are tiny chips of, for example, lithium are particularly preferable.
Double-layer pellets of this type cannot be manufactured by the conventional apparatus described above, however, due to the structure of the apparatus. To manufacture such double-layer pellets, chips used as cores may be made to float in air, and deuterium solidifies, forming outer layers on the chips. It is utmost difficult to manufacture the pellets by this method.